Process and amine-solvent absorbent for removing acidic gases from gaseous mixtures

ABSTRACT

Acidic gases are substantially removed from a normally gaseous mixture by a process comprising contacting the normally gaseous mixture with an amine-solvent liquid absorbent comprising (i) an amine mixture comprised of at least about 50 mol % of a sterically hindered amine and at least about 10 mol % of a tertiary amino alcohol, wherein said sterically hindered amine contains at least one secondary amino group which is part of a ring and is attached to either a secondary or tertiary carbon atom or a primary amino group attached to a tertiary carbon atom, and (ii) a solvent for said amine mixture which is also a physical absorbent for said acidic gases. The liquid absorbent may also include up to about 35 weight percent water. The coaction of the sterically hindered amine and the tertiary amino alcohol in conjunction with the solvent provides an increase in the amount of CO 2  containing acidic gases absorbed compared to the use of the same sterically hindered amines alone and lowers the heat of reaction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved process for removing carbondioxide containing acidic gases from normally gaseous mixtures containngthem and more particularly relates to a process of accomplishingsubstantial removal of these acidic gases from normally gaseous mixturesby contacting the normally gaseous mixtures with a solution containing asterically hindered amine, a tertiary amino alcohol and a solvent forsaid amines.

2. Description of the Prior Art

It is well known in the art to treat gases and liquids, such as mixturescontaining acidic gases including CO₂, H₂ S, SO₂, SO₃, CS₂, HCN, COS andoxygen and sulfur derivatives of C₁ to C₄ hydrocarbons with aminesolutions to remove these acidic gases. The amine usually contacts theacidic gases and the liquids as an aqueous solution containing the aminein an absorber tower with the aqueous amine solution contacting theacidic fluid countercurrently.

The acid gas scrubbing processes known in the art can be generallybroken into 3 categories.

The first category is generally referred to as the aqueous amine processwhere relatively concentrated amine solutions are employed during theabsorption. This type of process is often utilized in the manufacture ofammonia where nearly complete removal of the acid gas, such as CO₂, isrequired. It is also used in those instances where an acid gas, such asCO₂, occurs with other acid gases or where the partial pressure of theCO₂ and other gases are low.

A second category is generally referred to as the aqueous base scrubbingprocess or "hot pot" process. In this type of process a small level ofan amine is included as an activator for the aqueous base used in thescrubbing solution. This type of process is generally used where bulkremoval of an acid gas, such as CO₂, is required. This process alsoapplies to situations where the CO₂ and feed gas pressures are high. Insuch processes, useful results are achieved using aqueous potassiumcarbonate solutions and an amine activator.

A third category is generally referred to as the nonaqueous solventprocess. In this process, water is a minor constituent of the scrubbingsolution and the amine is dissolved in the liquid phase containing thesolvent. In this process, up to 50% of the amine is dissolved in theliquid phase. This type of process is utilized for specializedapplications where the partial pressure of CO₂ is extremely high and/orwhere many acid gases are present, e.g., COS, CH₃ SH and CS₂.

The present invention pertains to an improved process for practicing thethird category of the acid gas scrubbing process described above,namely, the nonaqueous solvent process where up to 50% or more of thesame absorbent is dissolved in the liquid phase containing a solvent forthe amine.

Many industrial processes for removal of carbon dioxide containingacidic gases use regenerable solutions of amines which are continuouslycirculated between an absorption zone where the acidic gases, e.g.,carbon dioxide, are absorbed and a regeneration zone where the aminecontaining absorption solution which is saturated with the acidiccomponents is desorbed usually by steam stripping. The capital cost ofthese acid gas scrubbing processes is generally controlled by the sizeof the absorption and regeneration towers, the size of the reboilers forgenerating stripping steam, and the size of the condensers whichcondense spent stripping steam so that condensate may be returned to thesystem to maintain proper water balance.

The cost of operating such scrubbing plants is generally related to theamount of heat required for the removal of a given amount of acid gas,e.g., thermal efficiency, sometimes expressed as cubic feet of acid gasremoved per pound of steam consumed. Means for reducing the costs inoperating these industrial processes have focused on the use ofabsorbing systems or combinations of chemical absorbents which willoperate more efficiently and effectively in acid gas scrubbing processesusing existing equipment.

There are a number of patents which describe improvements to improve theefficiency of the above-described processes for removing acidic gasesfrom gaseous mixtures. Some of these improvements are described below.

U.S. Pat. No. 2,360,861 teaches the use of cyclic tetramethylenesulfones for separating mixtures of organic compounds and U.S. Pat. Nos.2,385,704 and 3,475,329 teach the extraction of SO₂ withcyclotetramethylene sulfones.

U.S. Pat. No. 3,039,251 to Kamlet teaches the use of certain sulfones,such as cyclotetramethylene sulfone (i.e., sulfolane) and the homologuesthereof, alone or in combination with various alkanolamines for removalof hydrogen sulfide, mercaptans and/or carbon dioxide from normallygaseous mixtures. According to this patent, one is enabled to purify anddehydrate natural and synthetic gases, such as natural gas, synthesisgas, producer gas, coke oven gas, etc. The amines disclosed in thispatent include monoethanolamine, diethanolamine, triethanolamine,N-methyldiethanolamine and alpha-aminopropionic acid.

A number of improvements and variations on the process taught by U.S.Pat. No. 3,039,251 have appeared in the patent literature. Theseimprovements generally involve the use of specific amines or classes ofamines, addition of other additives, such as iodine or water, and theuse of other solvents, such as amides in place of the sulfones, etc.Examples of the patents which disclose some of these improvements are asfollows:

    ______________________________________                                        U.S. Pat. Nos.                                                                3,161,461        3,642,431                                                    3,347,621        3,653,809                                                    3,352,631        3,656,887                                                    3,363,989        3,656,905                                                    3,377,138        3,658,462                                                    3,463,603        3,681,015                                                    3,502,428        3,716,620                                                    3,532,467        3,719,749                                                    3,551,102        3,764,665                                                    3,551,106        3,767,766                                                    3,553,936        3,777,010                                                    3,565,573        3,801,708                                                    3,577,221        3,843,512                                                    3,618,331        3,928,548                                                    3,630,666        3,965,244                                                    3,632,519        3,965,253                                                    British Patent Specification Numbers                                            957,260        1,131,989                                                      972,140        1,153,786                                                      997,169        1,158,976                                                    1,058,304        1,238,696                                                    1,118,687                                                                     Canadian Patent Numbers                                                         951,494                                                                       729,090                                                                     Dutch Patent Specification Numbers                                             67/06653                                                                      73/12490                                                                      73/12491                                                                     German Offenlegungschrift Numbers                                             1,542,415                                                                     2,422,581                                                                     2,433,078                                                                     ______________________________________                                    

None of these patents or patent publications disclose, teach or suggestthe use of sterically hindered amines and a tertiary amino alcohol witha solvent such as sulfolane or the unexpected benefits of the instantinvention described herein. British patent specification Nos. 972,140,1,058,304 and 1,238,696 and U.S. Pat. No. 3,716,620, however, are ofparticular interest with respect to the instant invention and,accordingly, are discussed in more detail hereinafter.

British patent specification No. 972,140 (which generally corresponds toU.S. Pat. No. 3,161,461) discloses a process for removing acid gasesfrom gaseous streams by contacting the gaseous streams with acomposition comprising organic solvent, such as sulfolane, and at leastone amine having a weakly basic character in the range of pK_(b) at 25°C of 3-14, e.g., diethanolamine.

British patent specification No. 1,058,304 describes a process forremoving acid gases from gaseous streams by contacting the gaseousstreams with an aqueous solution comprising sulfolane and a secondaryalkanolamine or morpholine or derivatives of morpholine, e.g.,2,6-dimethylmorpholine, 2,6-diethylmorpholine,2,3,5,6-tetraethylmorpholine, 2-methylmorpholine, 2-ethylmorpholine and2-methyl-6-ethylmorpholine. The patent specification does not teach theconcept of improved working capacity as defined hereinafter or the useof sterically hindered primary alkanolamines, and more importantly thepatent specification does not teach the use of a sterically hinderedamine in combination with a tertiary amino alcohol and a solvent asprovided by the instant invention.

British patent specification No. 1,238,696 discloses a process forremoving acid gases from gaseous streams by contacting the gaseousstreams with a composition comprising an organic solvent and analkanolamine such as cyclohexylamino ethanol. Sterically hindered aminesas defined herein are not specifically taught in this patentspecification.

U.S. Pat. No. 3,716,620 discloses a process for removing acid gases fromgaseous streams by contacting the gaseous streams with a compositioncomprising a solution of iodine in an organic solvent which alsocontains an amine. Among the solvents disclosed, there are included thesulfones, e.g., sulfolane. Among the amines, there are disclosedalkanolamines with primary, secondary or tertiary amino groups having 2to 12 carbon atoms and 1 to 3 amine groups, e.g., mono, di- andtriethanolamines, N-methyl diethanolamine, N-cyclohexyldipropanolamineand diisopropanolamine.

Prior art workers have taught that sterically hindered amines would havelow rates of combination with CO₂ and apparently concluded, althoughother explanations are possible, that such sterically hindered amineswould be inefficient in CO₂ scrubbing processes. For example, Sharma,M.M., Trans. Faraday Soc., 61, 681-8 (1965) described the kinetics ofreaction between CO₂ and COS with 38 amines, some of which aresterically hindered amines. Other researchers have attributed relativelypoor absorption rates of CO₂ by amines to steric hindrance. See, forexample, J. L. Frahn and J. A. Mills, Aust. J. Chem., 17, 256-73, 263(1964) and M. B. Jensen, Acta Chemica Scandinavica, 11, 499-505 (1957).

Shrier and Danckwerts, Ind. Eng. Chem. Fundamentals, 8, 415 (1969)discussed the use of amines as promoters for aqueous carbon dioxideabsorption solutions. However, these researchers only ran initialabsorption rate experiments and did not recognize the unique capacityadvantages obtained by using sterically hindered amines in an acid gasscrubbing process. Also of interest is Danckwerts and Sharma, TheChemical Engineer, Oct. 1966, pp 244-280.

U.S. Pat. No. 2,176,441 to Ulrich et al, teaches the use of amino acidshaving a primary, secondary or tertiary amino group and at least twonitrogen atoms to remove acidic gases. The patentees provide variousgeneral formulae for the amino acids taught to be useful in the acid gasscrubbing process. While certain "sterically hindered amines" can bederived by proper choice of substituent groups in the general formulaethere is no teaching that these amines will achieve any unexpectedresults, such as improved regeneration rates coupled with high rates ofabsorption.

There are a number of patents which disclose the use of various aminesas "activators" in an alkaline scrubbing solution wherein the primaryabsorbent is an alkaline salt such as potassium carbonate. Some of theseprocesses are described in U.S. Pat. Nos. 2,718,454, 3,144,301,3,637,345, 3,793,434, 3,848,057, 3,856,921, 3,563,695, 3,563,696 and3,642,430, as well as some other patents such as Belgian Pat. No.767,105; British Pat. Nos. 1,063,517, 1,218,083 and 1,305,718.

In the prior art processes discussed above, it is apparent that theefficiency or processes employing absorbing solutions is generallylimited by the relatively slow rate of transfer of molecules of the acidgas from the gas phase to the liquid phase as well as in theregeneration of the absorbing solution. Many of the above-describedprior art processes deal with means to render the acid gas scrubbingprocess more efficient.

In copending U.S. application Ser. No. 590,427, filed June 26, 1975, thedisclosure of which is incorporated herein by reference, there isdisclosed and claimed sterically hindered amine compositions useful forscrubbing acid gases. These sterically hindered amines unexpectedlyimprove the efficiency, effectiveness and working capacity of the acidgas scrubbing processes in all three of the abovementioned processcategories. It was postulated in U.S. Ser. No. 590,427 that the increasein cyclic capacity observed with the sterically hindered amines is dueto the instability of their carbamates. In that respect, stericallyhindered amines are similar to tertiary amines. Tertiary amines are notused on a commercial scale for carbon dioxide containing acid gasscrubbing due to their low rates of absorption and desorption.

It has been observed that tertiary amino alcohols can replace a portionof the sterically hindered amines in the amine-solvent process disclosedin U.S. Ser. No. 590,427, and the advantages of improved cyclic capacityare still enjoyed. This advantage provides a reduction in the totalamine cost owing to the lower cost of the tertiary amines compared tothe sterically hindered amines.

It has also been discovered that the combination of tertiary aminoalcohol and the sterically hindered amines provides a lower heat ofreaction than the amine-solvent solution containing the stericallyhindered amines alone.

SUMMARY OF THE INVENTION

In one embodiment of the present invention there is provided a processfor the substantial removal of acidic gases from a normally gaseousmixture, which comprises contacting said normally gaseous mixture withan amine-solvent liquid absorbent comprising:

(i) an amine mixture comprised of at least 50 mol % of a stericallyhindered amine and at least about 10 mol % of a tertiary amino alcohol,wherein said sterically hindered amine contains at least one secondaryamino group which is part of a ring and is attached to either asecondary or tertiary carbon atom or a primary amino group attached to atertiary carbon atom, and

(ii) a solvent, preferably an organic solvent for said amine mixturewhich is a physical absorbent for said acidic gases.

In another embodiment of the present invention, there is provided anamine-solvent liquid absorbent comprising:

(a) an amine mixture comprised of:

(i) at least about 50 mol % of a sterically hindered amine and at leastabout 10 mol % of a tertiary amino alcohol, wherein said stericallyhindered amine contains at least one secondary amino group which is partof a ring and is attached to either a secondary or tertiary carbon atomor a primary amino group attached to a tertiary carbon atom, and

(b) a solvent for said amine mixture.

The liquid absorbent composition of the present invention may optionallycontain up to about 35 weight % of water, preferably up to 25 weight %water and more preferably 10 to about 20 weight % water. The water inthe liquid absorbent is used to generate steam (to help the heat balanceof the overall process), reduce the viscosity of the solvent,particularly sulfolane and glycol type solvents) and assist thesolubility of some amine-acid gas reaction products.

The terms "solvent" and "organic solvent" as used herein are meant toinclude those materials which appear to act in a purely physical manner,absorbing acidic gases physically without the formation of any apparentreaction product. The terms "solvent" and "organic solvent" are to becontrasted with the so-called "chemical solvents" which involve theformatipon of salts or other decomposable reaction products with acidicgases.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term "acidic gases" is meant to include CO₂, H₂ S, SO₂, SO₃, CS₂,HCN, COS and the oxygen and sulfur derivatives of C₁ to C₄ hydrocarbonsin various amounts as they frequently appear in gaseous mixtures. Theseacidic gases which are generally CO₂, H₂ S and COS may be present intrace amounts within a gaseous mixture or in major proportions, themajor proportion preferably being CO₂ for the purpose of the presentinvention.

The contacting of the amine-solvent liquid absorbent and the acidicgases may take place in any suitable contacting tower. In suchprocesses, the normally gaseous mixture from which the acidic gases areto be removed may be brought into intimate contact with the absorbingsolution using conventional means, such as a tower packed with, forexample, ceramic rings or saddles or with bubble cap plates or sieveplates, or a bubble reactor.

In a preferred mode of practicing the invention, the absorption step isconducted by feeding the normally gaseous mixture into the base of thetower while fresh and/or regenerated absorbing solution (i.e., theliquid absorbent of the present invention) is fed into the top. Thenormally gaseous mixture freed largely from acidic gases emerges fromthe top. Preferably, the temperature of the absorbing solution duringthe absorption step is in the range from about 20° to about 100° C, andmore preferably from 40° to about 60° C. The pressure of the normallygaseous mixture feed will preferably be in the range from about 1 toabout 200 psig, and more preferably in the range from about 100 to about1000 psig. The contacting takes place under conditions such that theacidic gases, e.g., CO₂ possibly in combination with H₂ S and/or COS areabsorbed by the solution. During absorption the solution is maintaied ina single phase.

The liquid absorbent composition comprising the amine mixture andsolvent which is saturated or partially saturated with gases, such asCO₂ and H₂ S, may be regenerated so that it may be recycled back to theabsorber. The regeneration should also take place in a single liquidphase. The regeneration or desorption is accomplished by conventionalmeans, such as pressure reduction, which causes the acid gases to flashoff or by passing the solution into a tower of similar construction tothat used in the absorption step, at or near the top of the tower, andpassing an inert gas such as air or nitrogen or preferably steam up thetower. The temperature of the solution during the regeneration stepshould be in the range from about 50° to about 170° C, and preferably80° to 150° C. The absorbing solution, after being cleansed of at leasta portion of the acid bodies, may be recycled back to the absorbingtower. Makeup absorbent may be added as needed.

For example, during desorption, the acidic gases, e.g., CO₂ possibly incombination with H₂ S and/or COS solutions from the high pressureabsorber is sent first to a flash chamber where steam and some CO₂ andthe H₂ S and/or COS are flashed from solution at low pressure. Theamount of CO₂ containing acidic gases flashed off will in general beabout 35 to 40% of the net CO₂ containing acidic gases recovered in theflash and stripper. This is increased somewhat, e.g., to 40 to 50%, withthe high desorption rate amine blend owing to a closer approach toequilibrium in the flash. Solution from the flash drum is then steamstripped in the packed or plate tower, stripping steam having beengenerated in the reboiler in the base of the stripper. Pressure in theflash drum and stripper is usually 1 to about 50 psia, preferably 15 toabout 30 psia, and the temperature is in the range from about 50° toabout 170° C, preferably 80° to about 150° C. Stripper and flashtemperature will, of course, depend on stripper pressure, thus at about15 to 30 psia stripper pressures, the temperature will be about 80° toabout 150° C during desorption.

In the most preferred embodiment of the present invention, substantiallycomplete removal of carbon dioxide containing acidic gases from normallygaseous mixtures is accomplished by a process comprising:

(a) contactng said normally gaseous mixture with a liquid absorbentcomposition, said liquid absorbent composition comprising:

i. an amine mixture comprising at least 50 mol %, preferably at least 65mol %, of a sterically hindered amino alcohol and at least about 10 mol%, and preferably 20 to about 35 mol % of a tertiary amino alcohol,wherein said sterically hindered amino alcohol contains at least onesecondary amino group which is part of a ring and is attached to eithera secondary or tertiary carbon atom or a primary amino group attached toa tertiary carbon atom, wherein the total amine concentration in saidliquid absorbent is in the range from about 10 weight % to about 65weight %, preferably from 40 weight % to about 55 weight %, and

(ii) an organic solvent which is a solvent for said amine mixture and aphysical absorbent for said acidic gases, said contacting beingconducted at conditions whereby the carbon dioxide containing acidicgases are absorbed, preferably at temperatures ranging from about 20° toabout 100° C, and more preferably from 40° to about 60° C, and at apressure ranging from about 1 to about 2000 psig, preferably 100 toabout 1000 psig, and

(iii) water, in an amount ranging from 1 to 35 weight %, preferably 10to 20 weight %, and

(b) regenerating the liquid absorbent composition by a desorption stepat conditions whereby at least a portion of said acidic gases aredesorbed from the liquid absorbent, preferably at a temperature in therange from about 50° to about 170° C, and more preferably from 80° toabout 150° C and at a pressure ranging from about 1 to about 50 psia andmore preferably from about 15 to 30 psia.

The regenerated liquid absorbent composition may thereafter be recycledto the absorber as is or it may be combined with fresh makeup scrubbingsolution.

THE STERICALLY HINDERED AMINES

As stated earlier, the sterically hindered amines are preferablysterically hindered amino alcohols which include those amino alcoholswhich contain at least one secondary amino group which is part of a ringand is attached to either a secondary or tertiary carbon atom or aprimary amino group attached to a tertiary carbon atom. The mostpreferred sterically hindered amino alcohols useful in the practice ofthe present invention include the 2-piperidine alkanols such as2-piperidine methanol and 2-piperidine ethanol. Other amines that can beused include 2-(1-hydroxyethyl)-piperidine, 5-hydroxy-2-methylpiperidine, 2-methyl-3-hydroxy piperidine, 2,6-dimethyl-3-hydroxypiperidine and 2,5-dimethyl-4-hydroxy piperidine,3-amino-3-methyl-1-butanol, 2-amino-2-methyl-1-butanol,2-amino-2-methyl-3-butanol and 3-amino-3-methyl-2-pentanol.

THE TERTIARY AMINO ALCOHOLS

The tertiary amino alcohols to be used to replace a portion of thesterically hindered amine in the liquid absorbent include those tertiaryamino alcohols having 4 to 8 carbon atoms and contain from 1 to 3hydroxyl groups. Preferred tertiary amino alcohols include3-dimethylamino-1-propanol, 1-diethylamino-2-propanol,2-diethylaminoethanol, 3-diethylamino-1-propanol, N-methyldiethanolamine, and 1-methyl-3-hydroxy piperidine.

THE SOLVENT

The solvents useful in the practice of the present invention arepreferably organic compounds which will (1) contain at least onefunctional group to solubilize the amines; (2) be unreactive with theamines; (3) be a liquid at room temperature; (4) have a solubility forCO₂ at 25° C and one atmosphere of not less than about one volume of CO₂per volume of solvent; and (5) have a boiling point at atmosphericpressure of at least about 150° C. Suitable organic solvents useful inthe practice of the present invention include sulfones; sulfoxides;glycols and the mono- and diethers thereof; 1,3-dioxo compoundscharacterized by being a 5- to 6-membered heterocyclic ring, e.g.,dioxolanes and dioxanes; aromatic ethers; aromatic hydrocarbons;pyrrolidones; piperidones; and mixtures thereof.

A specific description of te organic solvents useful in the practice ofthe present invention is as follows:

The sulfones useful as solvents in the practice of the present inventionare based upon cyclotetramethylene sulfone, the basic and preferredspecies being sulfolane and otherwise known as thiophenetetrahydro-1,1-dioxide. The sulfones have the general formula: ##STR1##wherein at least 4 of the R substituents are hydrogen radicals and anyremaining Rs being alkyl groups having from 1 to 4 carbon atoms. It ispreferred that no more than 2 alkyl substituents are appended to thetetramethylene sulfone ring.

Suitable sulfone derivatives include 2-sulfolene; 2-methyltetramethylene sulfone; 3-methyl tetramethylene sulfone; 2,3-dimethyltetramethylene sulfone; 2,4-dimethyl tetramethylene sulfone;3,4-dimethyl cyclotetramethylene sulfone; 2,5-dimethylcyclotetramethylene sulfone; 3-ethyl cyclotetramethylene sulfone;2-methyl-5-propyl cyclotetramethylene sulfone as well as their analoguesand homologues.

The sulfoxides useful as solvents in the practice of the presentinvention include the alkyl-, cycloalkyl- or arylsulfoxides, forexample, those sulfoxides of the general formula:

    R.sub.9 -- SO -- R.sub.10

wherein R₉ and R₁₀ are the same or different hydrocarbon monovalentradicals, for example, alkyl radicals having from 1 to 20 and preferablyfrom 1 to 3 carbon atoms, cycloalkyl radicals having for example, from 3to 20 carbon atoms or aryl radicals having, for example, from 6 to 20carbon atoms. The R₉ and R₁₀ radicals may also be joined together toform an alkylene radical preferably containing 4 to 12 carbon atoms,thus resulting in a ring comprising the sulfur atom.

The following compounds are nonlimitative examples of the sulfoxideswhich are useful solvents: dimethyl sulfoxide, diethyl sulfoxide,dipropyl sulfoxide, dibutyl sulfoxide, methylethyl sulfoxide,dicyclohexyl sulfoxide, methylcyclohexyl sulfoxide, diphenyl sulfoxide,ethylphenyl sulfoxide, cyclohexylphenyl sulfoxide and tetramethylenesulfoxide. Dimethyl sulfoxide is the most preferred sulfoxide to be usedas a solvent for the liquid absorbents of the invention.

The glycols, polyethylene glycols, polyalkylene glycols and their mono-and diethers useful in the practice of the present invention include thecompounds of the following general formula:

    R.sub.11 --O--A--0--.sub.m R.sub.12

wherein A is an alkylene radical which is either linear or branched,having from 2 to 15 carbon atoms and preferably 2-5 carbon atoms; R₁₁and R₁₂ are the same or different, e.g., hydrogen atoms, hydrocarbonmonovalent radicals having, for example, from 1 to 20 carbon atoms, forexample, alkyl, cycloalkyl or aryl radicals; m is an integer of 1 to 20and preferably 1 to 10.

The following compounds are nonlimiting examples of the glycols,polyethylene glycols, polyalkylene glycols and their mono- and diethersuseful as solvents for the liquid absorbents: glycol; diethylene glycol;heptaethylene glycol; decaethylene glycol; 1,3-propylene glycol;hepta(1,3-propylene glycol), tetra-(1,4-butylene glycol), polyethyleneglycol of abut 400 molecular weight; and tri(1,3-propylene glycol). Thepreferred compounds of this class are the polyethylene glycols and theirmonoalkyl ethers.

The 1,3-dioxoheterocyclic compounds useful in the practice of thepresent invention can be represented by the following general formulae:##STR2## wherein R₁₃ to R₁₈ represent hydrogen, lower alkyl groupscontaining 3 to 5 carbon atoms, lower alkyl groups substituted with OH,OR, or phenyl groups, or unsubstituted phenyl groups so selected thatthe molecular ratio of carbon atoms to oxygen atoms is between 1 and 10.Typical compounds of this class which may be used in accordance with thepractice of the present invention include:2,2-dimethyl-1,3-dioxolane-4-methanol;2-methyl-2-ethyl-1,3-dioxolane-4-methanol;2-methyl-2-ethyl-4-methoxymethyl-1,3-dioxolane;2,2,4-trimethyl-1,3-dioxane; 1,4-dioxaspiro (4,4) nonane;2-hydroxymethyl-1,4-dioxaspiro (4,4) nonane; and 1,4-dioxaspiro (4,5)decane.

The aromatic hydrocarbons and aromatic ethers useful in the practice ofthe present invention include those compounds which are liquid at theabsorption temperatures, for example, benzene, toluene, orthoxylene,metaxylene, ethylbenzene, paraethyl toluene, diphenyl ether, and theirhomologues having up to 12 carbon atoms.

The pyrrolidones and piperidones useful in the practice of the presentinvention include the N-alkyl pyrrolidones and the N-alkyl piperidoneshaving 4 to 12 carbon atoms, for example, 2-pyrrolidone, N-methylpyrrolidone, N-ethyl pyrrolidone and N-methyl piperidone.

The organic solvent component of the liquid absorbents of the presentinvention may include a mixture of two or more of the compoundsdescribed above.

THE LIQUID ABSORBENT COMPOSITION

The design of the specific composition to be used for the acid gasscrubbing process of the present invention will be based upon the typeand amount of acidic gases in the normally gaseous mixture to bescrubbed by the liquid absorbent composition of the present invention.In treating normally gaseous mixtures having a high concentration ofcarbon dioxide and hydrogen sulfide, e.g., 35% CO₂ and 10- 12% H₂ S, aliquid absorbent having a relatively high amine concentration should beused. Since it is important that the reaction product of the acidicgases and the amines remain soluble, the water content should beincreased with correspondingly increased concentration of the aminemixture. Therefore, a typical composition useful for scrubbing anormally gaseous mixture having a high concentration of carbon dioxideand hydrogen sulfide will be comprised of about 50-60 weight % of theamine mixture, 1 to 15 weight % of at least one organic solvent,preferably sulfolane, and the balance water.

When the normally gaseous mixture to be treated contains a significantamount of organo sulfur compounds, e.g., mercaptans in amounts of about6% or more, the liquid scrubbing composition of the present inventionshould be designed to have a high concentration of the solvent so as tosolubilize the organo sulfur compounds (the organo sulfur compounds arerelatively insoluble in aqueous solutions, but are solubilized byorganic solvents such as sulfolane). A typical liquid scrubbing solutionfor use in removing large amounts of organo sulfur compounds fromnormally gaseous mixtures should include about 15 weight % of the aminemixture, about 75 weight % of at least one organic solvent, preferablysulfolane, and the balance, about 10 weight % water.

In those instances where carbon dioxide is the major part of the acidicgases and hydrogen sulfide and COS exist as minor components of theacidic gases in the normally gaseous mixture to be treated, it isdesirable to use a liquid scrubbing composition having nearly anequivalent weight % of the amine mixture and organic solvent andsufficient water to generate steam and reduce the viscosity of thesolvent. A typical liquid scrubbing solution for use in scrubbing thesehigh CO₂ content gases will contain about 45 weight % of the aminemixture, about 40 weight % of the solvent, preferably sulfolane, and thebalance, about 15 weight % water.

As can be seen from the above considerations, the amount of the aminemixture, solvent and optionally water to be used to prepare the liquidabsorbent scrubbing composition of the present invention may varywidely, depending on the intended use and the type of gas to be treated.Generally speaking, one typical liquid absorbent composition of thepresent invention will contain from 10 to about 45 weight % of the aminemixture, 1 to about 35 weight % water and the balance being at least oneorganic solvent. Another typical liquid absorbent composition will becomprised of 45 to about 65 weight % of the amine mixture, 15 to about35 weight % water and essentially all of the balance being at least oneorganic solvent for the amine mixture.

In these liquid absorbent compositions as described above, the aminemixture will contain at least about 50 mol %, preferably 65 mol % of thesterically hindered amine, preferably a sterically hindered aminoalcohol and at least about 10 mol %, preferably 20 to about 35 mol % ofthe tertiary amino alcohol.

The normally gaseous mixture to be treated in accordance with theprocess of the present invention may include sour natural gas, hydrogensynthesis gas, refinery gas or any other normally gaseous mixturecontaining acidic gases.

The amine mixture may be used with the solvent and optionally water inany convenient manner. In one preferred embodiment of the invention asterically hindered amino alcohol and tertiary amino alcohol arepremixed with the solvent (and optionally water) and placed in the formof a concentrate. This concentrate may optionally include commonly usedadditives, such as antifoaming agents, antioxidants, corrosioninhibitors, etc. Examples of such additives include arsenious anhydride,selenious and tellurous acid, protides, vanadium oxides, e.g., V₂ O₃,chromates, e.g., K₂ Cr₂ O₇, iodine and iodine compounds, etc.

It is possible, of course, to employ the process of the presentinvention in conjunction with other acid gas scrubbing processes. Forexample, solutions rich in carbon dioxide may be first scrubbed by abulk scrubbing process using the "hot pot" process, preferably theprocesses disclosed in U.S. Ser. No. 590,427 and U.S. Ser. No. 750,520,filed Dec. 15, 1976, entitled "Process for Removing Acid Gases withHindered Amines and Aminoacids," the disclosures of which areincorporated herein by reference. This coarsely prepurified gas may thenbe treated in accordance with the teachings of the present invention toremove the last residues of the carbon dioxide containing gases.

The invention is illustrated further by the following examples which,however, are not to be taken as limiting in any respect. All parts andpercentages, unless expressly stated to be otherwise, are by weight.

EXAMPLE 1

This example is carried out for comparison purposes only.

The reaction apparatus consists of an absorber and a desorber. Theabsorber is a glass vessel having a capacity of 2.5 liters and adiameter of 10 cm equipped with a heating jacket. The stirrer shaftcarries two 3-blade propellers, of which the upper one pushes the liquiddownward and the lower one pushes the liquid upward. A pump removesliquid from the bottom of the reactor and feeds it back above the liquidthrough a stainless steel sparger. The apparatus can be evacuated.Nitrogen and CO₂ can be fed to the bottom of the cell through a sparger.

The desorber is a 1-liter glass reactor, equipped with stirrer, gassparger, reflux condenser and thermometer.

The following reagents are charged into the absorber:

324 g of 2-piperidine ethanol

300 g of tetrahydrothiophene-1,1-dioxide (sulfolane)

112.5 g of water

The solution is brought to 40° C, then the reactor is evacuated and CO₂is admitted into it. 36 liters of CO₂ are absorbed. The rich solution soobtained is transferred to the desorber, where it is heated at 105° Cfor 15 minutes. The regenerated solution so obtained is transferred backto the absorber and subjected again to absorption. 31.9 liters of CO₂are absorbed.

A wet-test meter inserted between the CO₂ cylinder and the absorberpermits to establish the amounts of gas absorbed as a function of time.The result is given in Table I.

                  TABLE I                                                         ______________________________________                                                       Time                                                           Liters of CO.sub.2 Absorbed                                                                    Min.        Sec.                                             ______________________________________                                         5               0           13                                               10               0           31                                               15               0           53                                               20               1           24                                               25               2           21                                               30               5           02                                               ______________________________________                                    

EXAMPLE 2

A new solution is prepared, in which one-third of the piperidine ethanolis replaced by an approximately equivalent amount of a tertiary amine.The solution has the following composition:

215 g of 2-piperidine ethanol

97 g of 3-dimethylamino-1-propanol

300 g of tetrahydrothiophene-1,1-dioxide (sulfolane)

112.5 g of water

The solution is subjected to the same cycle as before, i.e., absorptioninto fresh solution, desorption and absorption into lean solution. TableII gives the amounts of CO₂ absorbed into lean solution as a function oftime. The total amount absorbed into the lean solution is 32 liters.

                  TABLE II                                                        ______________________________________                                                       Time                                                           Liters of CO.sub.2 Absorbed                                                                    Min.        Sec.                                             ______________________________________                                         5               0           13                                               10               0           31                                               15               0           53                                               20               1           27                                               25               2           27                                               30               4           54                                               ______________________________________                                    

Comparison with Table I shows that the two solutions have a very similarbehavior.

EXAMPLE 3

The following solution is prepared;

215 g of 2-piperidine ethanol

109 g of 1-diethylamino-2-propanol

300 g of tetrahydrothiophene dioxide (sulfolane)

112.5 g of water

Compared to the solution of Example 1, in this solution one-third of thepiperidine ethanol has been replaced by an equivalent amount of atertiary amino alcohol. The solution is subjected to the same cycle asbefore, i.e., absorption into fresh solution, desorption and absorptioninto lean solution. Table III gives the amounts of CO₂ absorbed intolean solution as a function of time. The total amount absorbed into thelean solution is 29.5 liters.

                  TABLE III                                                       ______________________________________                                                       Time                                                           Liters of CO.sub.2 Absorbed                                                                    Min.        Sec.                                             ______________________________________                                         5               0           11                                               10               0           27                                               15               0           41                                               20               1           21                                               25               2           30                                               ______________________________________                                    

Comparison with Table I shows that the two solutions have a similarbehavior.

EXAMPLE 4

The following solution is prepared:

215 g of 2-piperidine ethanol

87 g of 2-diethylamino ethanol

300 g of tetrahydrothiophene dioxide (sulfolane)

112.5 g of H₂ O

Compared to the solution in Example 1, in this solution one-third of thepiperidine ethanol has been replaced by an approximately equivalentamount of a tertiary amino alcohol. The solution is subjected to thesame cycle as before, i.e., absorption into fresh solution, desorptionand absorption into lean solution. Table IV gives the amount of CO₂absorbed as a function of time. The total amount absorbed into the leansolution is 29 liters.

                  TABLE IV                                                        ______________________________________                                                       Time                                                           Liters of CO.sub.2 Absorbed                                                                    Min.        Sec.                                             ______________________________________                                         5               0           18                                               10               0           30                                               15               0           50                                               20               1           21                                               25               2           31                                               ______________________________________                                    

Comparison with Table I shows that the two solutions have a similarbehavior.

The process of the present invention, in addition to providing theadvantages of an improved lower heat of reaction and a lower cost aminemixture compared to the use of sterically hindered amines alone, alsoenjoys the advantage of improved "working capacity" compared to aprocess operating under substantially the same conditions without theuse of a sterically hindered amine as disclosed and claimed in U.S. Ser.No. 590,427, filed June 26, 1975, the disclosure of which isincorporated herein by reference. As in the case of U.S. Ser. No.590,427, the term "working capacity" relates to the thermodynamic cycliccapacity, that is the loading as measured at equilibrium conditions.This working capacity may be obtained from the relation between the CO₂pressure in the gas and the CO₂ loading in the solution at equilibriumat a given temperature. To calculate the thermodynamic cyclic capacity,the following parameters must usually be specified: (1) CO₂ absorptionpressure, (2) CO₂ regeneration pressure, (3) temperature of absorption,(4) temperature of regeneration, (5) solution composition, that isweight percent amine, and (6) gas composition.

The improved working capacity which results by the use of the stericallyhindered amine and the tertiary amino alcohol in the solvent andoptionally water can be determined by direct comparison with a processwherein a sterically hindered amine is not included in the amine-solventscrubbing liquid. For example, it will be found when comparing twoamine-solvent CO₂ scrubbing processes (that is similar gas composition,similar scrubbing solution, similar pressure and temperature conditions)that when the sterically hindered amines and tertiary amino alcohols areutilized the difference between the amount of CO₂ absorbed at the end ofthe absorption step (at equilibrium) and desorption (at equilibrium) issignificantly greater, generally at least 15% greater, and often 20 to60% greater than the working capacity of an amine-solvent scrubbingliquid which does not include a sterically hindered amine and a tertiaryamino alcohol, e.g., an amine-solvent liquid containingdiisopropanolamine.

Accordingly, in another preferred embodiment of the present invention, acarbon dioxide containing acidic gas is removed from a normally gaseousstream by means of a process which comprises, in sequential steps, (1)contacting the normally gaseous feed stream with an aminesolvent liquidabsorbent at conditions whereby said carbon dioxide containing gas isabsorbed in said liquid absorbent, and (2) regenerating said liquidabsorbent at conditions whereby said acid gas is desorbed from saidliquid absorbent, the improvement which comprises operating said processat conditions whereby the working capacity is greater than obtainedunder substantially the same conditions of absorption and desorptionexcept that said liquid absorbent does not include a sterically hinderedamine and a tertiary amino alcohol, wherein said working capacity isdefined as the difference in moles between CO₂ loading in the liquidabsorbent at absorption conditions (step 1) and the CO₂ loading in theliquid absorbent at regeneration conditions (step 2) when measured atequilibrium conditions.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth, and as fall within the scope of theinvention.

What is claimed is:
 1. A process for the removal of acidic gases from anormally gaseous mixture, which comprises contacting said normallygaseous mixture with an amine-solvent liquid absorbent being comprisedof:(i) an amine mixture comprised of at least 50 mol % of a stericallyhindered amine and at least about 10 mol % of a tertiary amino alcohol,wherein said sterically hindered amine contains at least one secondaryamino group which is part of a ring and is attached to either asecondary or tertiary carbon atom or a primary amino group attached to atertiary carbon atom, and (ii) a solvent for said amine mixture which isa physical absorbent for said acidic gases.
 2. The process of claim 1wherein said sterically hindered amine is a member selected from thegroup consisting of 2-piperidine methanol, 2-piperidine ethanol,2-(1-hydroxyethyl)-piperidine, 5-hydroxy-2-methyl piperidine,2-methyl-3-hydroxy piperidine, 2,6-dimethyl-3-hydroxy piperidine,2,5-dimethyl-4-hydroxy piperidine, 3-amino-3-methyl-1-butanol,2-amino-2-methyl-1-butanol, 2-amino-2-methyl-3-butanol and3-amino-3-methyl-2-pentanol.
 3. The process of claim 1 wherein thetertiary amino alcohol is a member selected from the group consisting of3-dimethylamino-1-propanol, 1-diethylamino-2-propanol, 2-diethylaminoethanol, 3-diethylamino-1-propanol, N-methyl diethanolamine and1-methyl-3-hydroxy piperidine.
 4. The process of claim 1 wherein saidsolvent is a member selected from the group consisting of sulfones;sulfoxides; glycols, mono- and diethers thereof; 1,3-dioxo compoundscharacterized by being a 5- to 6-membered heterocyclic ring; aromaticethers; aromatic hydrocarbons; pyrrolidones; piperidones; and mixturesthereof.
 5. The process of claim 1 wherein said sterically hinderedamine is a 2-piperidine alkanol.
 6. The process of claim 1 wherein saidsterically hindered amine is a member selected from the group consistingof 2-piperidine methanol and 2-piperidine ethanol.
 7. The process ofclaim 1 wherein said tertiary alkanolamine is a member selected from thegroup consisting of 3-dimethylamino-1-propanol,1-diethylamino-2-propanol and 2-diethylamino ethanol.
 8. The process ofclaim 1 wherein said solvent is a cyclotetramethylene sulfone.
 9. Theprocess of claim 1 wherein the solvent is sulfolane.
 10. The process ofclaim 1 wherein said liquid absorbent includes up to about 35 weightpercent of water.
 11. The process of claim 1 wherein said acidic gasesare absorbed by said liquid absorbent at a temperature ranging fromabout 20° to about 100° C and at a pressure ranging from 1 to about 2000psig, and said acidic gases are desorbed from said liquid absorbent at atemperature ranging from about 50° to about 170° C and at a pressureranging from about 1 to about 50 psia.
 12. The process of claim 11wherein the absorption is conducted at a temperature ranging from about40°to about 60° C and at a pressure ranging from about 100 to about 1000psig, and the desorption is conducted at a temperature ranging fromabout 80° to about 150° C and at a pressure ranging from about 15 toabout 30 psia.
 13. A process for the substantial removal of carbondioxide containing acidic gases from a normally gaseous mixture, whichcomprises:(a) contacting said normally gaseous mixture with anamine-solvent liquid absorbent to absorb said carbon dioxide containingacidic gases, wherein said amine-solvent liquid absorbent comprises:(i)an amine mixture comprises of at least 50 mol % of a 2-piperidinealkanol and at least 10 mol % of a tertiary amino alcohol which is amember selected from the group consisting of 3-dimethylamino-1-propanol,1-diethylamino-2-propanol and 2-diethylamino ethanol, (ii) sulfolane,and (iii) up to about 35 weight percent of water; wherein saidabsorption is conducted at a temperature ranging from about 40° to about60° C and at a pressure ranging from about 100 to about 1000 psig, and(b) desorbing at least a portion of the absorbed carbon dioxidecontaining acidic gases from said solution at a temperature ranging fromabout 80° to about 150° C and at a pressure ranging from about 15 toabout 30 psia.
 14. The process of claim 13 wherein said 2-piperidinealkanol is 2-piperidine ethanol.
 15. The process of claim 13 wherein thewater content of said amine-solvent liquid absorbent is in the rangefrom about 10 to about 20 weight percent.
 16. The process of claim 1wherein the aminesolvent liquid absorbent additionally includesadditives selected from the group consisting of antifoaming agents,antioxidants and corrosion inhibitors.
 17. The process of claim 13wherein the aminesolvent liquid absorbent additionally includesadditives selected from the group consisting of antifoaming agents,antioxidants and corrosion inhibitors.
 18. A process for the substantialremoval of carbon dioxide containing acidic gases from a normallygaseous mixture, which comprises:(a) contacting said normally gaseousmixture with an amine-solvent liquid absorbent to absorb said carbondioxide containing acidic gases, wherein said aminesolvent liquidabsorbent comprises:(i) an amine mixture comprised of at least 65 mol %of 2-piperidine ethanol and from 20 to about 35 mol % of a tertiaryamino alcohol selected from the group consisting of3-dimethylamino-1-propanol, 1-diethylamino-2-propanol and 2-diethylaminoethanol, wherein the total amine concentration in said amine-solventliquid absorbent is in the range from about 10 weight % to about 65weight %, (ii) water, in an amount ranging from about 10 to about 20weight %, and (iii) essentially all of the balance of the amine-solventliquid absorbent being comprised of sulfolane; wherein said absorptionis conducted at a temperature ranging from about 40° to about 60° C. andat a pressure ranging from about 100 to about 1000 psig, and; (b)desorbing at least a portion of the absorbed carbon dioxide containingacidic gases from said solution at a temperature ranging from about 80°to about 150° C. and at a pressure ranging from about 15 to about 30psia.
 19. The process of claim 18 wherein the aminesolvent liquidabsorbent from step (b) is recycled in the absorption process of step(a).
 20. The process of claim 18 wherein the aminesolvent liquidabsorbent additionally includes additives selected from the groupconsisting of antifoaming agents, antioxidants and corrosion inhibitors.